The International Cessna^® 170 Association

I'm doing an electrical load analysis for a field approval I am working on. I've read many posts on many forums alluding to the idea that landing/taxi lights are an "intermittent load" but I can't seem to find an FAA reference to that. Does anyone know where the FAA says that specifically or something contrary.

FWIW, my 185 service manual includes a list of electrical items and there current draw, and there is a separate section labeled "Items not considered part of running load", and in that list are the landing lights. With that in mind, I'm inclined to believe the tribal knowledge. If Cessna did it, it has a shot at being legal, right?

Lopez it is in 43.13 electrical. Chapter 11, section -36 Determining Electrical Load

HOLY COW I FOUND IT!!! Actually a little help from the BCP board got me there. The FAA's Airplane Flying Handbook, figure 16-9 attached...

Lopez, only for the purpose of clarifying the private msgs which we shared......that appears to be a publication-article...not a resource. (Authored by "El Capitan"?)
It is only an illustration of what types of loads might meet the definition of continuous versus intermittent. (I don't believe it's a regulatory document. Where did you source it?)

Also, as I mentioned in our private msgs, as a practical matter (if not a legal one) .... "cigarette lighters" fail to meet their "Intermittent" status when continuously used for power-supplies for "portable equipment".

The table provided may have other serious flaws. For example, the heater blower motor included in the intermittent table, might quickly drain a system which did not provide for it's more-likely continuous useage. And the ratings/loads may not be representative...for example, the (4) nav lites do not specify make/model/brand or actual lamps, etc..

(And since when do light single engine aircraft have FOUR wingtip nav lights? And where does that table account for anti-collision-lighting? Even if the FOUR wingtip lights were to include strobes....it would be incorrect. Either they aren't wingtip....or they aren't FOUR and still meet FAR requirements.)

Anyway, the private correspondence we enjoyed seemed to indicate you were searching for an "official" statement by FAA that landing/taxi lights were not continuous loads. I don't think the article posted meets that definition.

It seems the FAA definition of what is a continuous or intermittent load has been a bit elusive. I think I’ve found it. First, we find that the FAA has let the American Society for Testing and Materials (ASTM) make the rules in this regard. Federal Register / Vol. 71, No. 48

The FAA finds the following new consensus standard acceptable for normal ... category airplanes. The consensus standard listed below may be used unless the FAA publishes a specific notification otherwise.
a. ASTM Designation F 2490–05, titled: Standard Guide for Aircraft Electrical Load and Power Source Capacity Analysis.

So then we look at ASTM Designation F 2490–05. In paragraph 6.4.1.7 they divide operating time into three categories: The 5 second analysis would be considered momentary -- like discharging your parachute flares. The loads between 5 seconds and 5 minutes are considered intermittent -- dome light perhaps -- and finally, loads on longer than 5 minutes are considered continuous.

So how did the landing and taxi lights get the "intermittent" classification? The general rule/advice is to have them on within 10 miles of the airport and below 10,000' -- hardly less than five minutes. When our aircraft were built this was not the general rule. In 1973 the FAA came up with the "Operation Lights On" program as a collision avoidance strategy. I think this is when landing/taxi lights went from being intermittent loads to continuous loads -- in practice if not on paper. It looks like Cessna tried to strike a middle ground on this program by suggesting only using the taxi light as an anti-collision light when they issued SNL85-35. So Landing Light = Intermittent load, Taxi Light = Continuous load? Tough to do with one switch for both lights.